In general, TDD based cellular communication networks require strict time synchronization between network elements for proper performance. Examples of Such TDD based cellular communication networks include a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) network, a Worldwide Interoperability for Microwave Access (WiMAX) network, and a Long Term Evolution-Time Division Duplex (LTE-TDD) network.
Once a BS located in a cell in a TDD based cellular communication network loses synchronization, typically due to losing system timing such as a Global Positioning System (GPS) reference signal, it will generate a huge interference to its neighboring cells. As a result, some or all of its neighboring cells may be out of service. This is explained below in the context of a TD-SCDMA network as an example.
In a TD-SCDMA network, uplink transmission and downlink transmission are on the same frequency band and switched in time domain. A TD-SCDMA radio frame is 10 ms in length, which is divided into two radio subframes. Each of the two radio subframes is 5 ms in length and comprises seven regular time slots (TS0-TS6), and three special time slots including a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS), as schematically shown in FIG. 1A.
The DwPTS, the GP, and the UpPTS in length are respectively 75 μs, 75 μs, and 125 μs, which respectively correspond to 96 chips, 96 chips, and 160 chips. Each of the seven regular time slots is 675 μs in length and comprises 864 chips. As schematically shown in FIG. 1B, each regular time slot sequentially comprises a block of 352 chips for data symbols, a block of 144 chips for midamble code, a block of 352 chips for data symbols, and a block of 16 chips for a Guard Period (GP).
In general, the time slot TS0 is always allocated to downlink transmission, and the time slot TS1 is always allocated to uplink transmission. Depending upon traffic requirements, the time slots TS2-TS6 can be allocated to downlink transmission and uplink transmission. FIG. 1A shows only one of several allocation ways of the time slots TS2-TS6.
Further, each radio subframe has two switching points. A first switching point for switching from downlink transmission to uplink transmission is always located in the GP between the time slots TS0 and TS1 due to the above allocation of the time slots TS0 and TS1. A second switching point for switching from uplink transmission to downlink transmission can be located in the GP of the last uplink time slot of the time slots TS1-TS5. The second switching point schematically illustrated in FIG. 1A is located at the end of the time slot TS3 which is the last uplink time slot of the time slots TS1-TS5.
It can be known from above that the GP in which the first switching point is located is 75 μs in length (i.e. 96 chips) and the GP in which the second switching point is located is 12.5 μs in length (i.e. 16 chips). In view of this and the above structure of the radio subframe, an inter-cell cross-slot interference occurs in two situations.
Referring now to FIG. 2A, there is schematically illustrated one situation in which an inter-cell cross-slot interference is resulted from an out-of-sync Node-B. In the figure, Node-B I is in perfect synchronization, whereas Node-B II is out of synchronization, whose radio subframe lags by a time Δt. It can be seen that the inter-cell cross-slot interference will occur as long as the time Δt is above 75 μs (i.e. 96 chips). The interference portion in the radio subframe of Node-B I is schematically illustrated by a hatched area.
FIG. 2B schematically illustrates another situation in which an inter-cell cross-slot interference is resulted from an out-of-sync Node-B. In the figure, Node-B I is in perfect synchronization, whereas Node-B II is out of synchronization, whose radio subframe leads by a time Δt. It can be seen that the inter-cell cross-slot interference will occur as long as the time Δt is above 12.5 μs (i.e. 16 chips). The interference portion in the radio subframe of Node-B I is schematically illustrated by a hatched area.
In either case, the inter-cell cross-slot interference caused by the out-of-sync Node-B II results in a high Block Error Rate (BLER) of uplink reception of the synchronized Node-B I, mainly because the data symbols under the interference have a very high soft value which will adversely affect other soft values during decoding. In this way, the synchronized Node-B I may not operate properly□ and thus a cell served by the synchronized Node-B I may be out of service. In fact, the out-of-sync Node-B II may adversely affect all its neighboring cells in a similar way. Therefore, network operators desire to recognize an out-of-sync Node-B as soon as possible in order to take actions to ensure network stability.
Although a Chinese patent CN 100361556C discloses a method of locating a source of uplink interference, the method has a number of disadvantages. For example, the method determines the existence of uplink interference in a very rough manner. Only one threshold for Received Signal Strength Indication (RSSI) or Received Total Wideband Power (RTWP) is set to determine the existence of the uplink interference, which is applicable only to detection of very strong interference and is not flexible in an actual implementation.
Accordingly, it would be desirable to provide a method of and a network controller for recognizing an out-of-sync BS in a TDD based cellular communication network to overcome the above disadvantages.